[analyzer] Represent parameters of the entrypoint as ParamVarRegions

Currently the parameters of the entrypoint function are represented by `NonParamVarRegion`s which is confusing and counterintuitive. It would be nice to ensure that all parameters are represented by `ParamVarRegion`s -- but to achieve this, somebody must review the relevant parts of the code to ensure that this does not break anything.

This quirk surfaced e.g. at [this review comment](https://github.com/llvm/llvm-project/issues/214226#issuecomment-5232052999) but I also encountered it separately.

Obviously this is not high priority, but would be nice to fix eventually.

Start in the analyzer code that represents parameters of the entrypoint function, focusing on the distinction between NonParamVarRegion and ParamVarRegion. Review the relevant uses and the linked review discussion for assumptions about these regions. Done means entrypoint parameters consistently use ParamVarRegion without breaking existing behavior.

Written by the indexing model from the issue text.